This invention relates generally to novel photoactive compounds and methods for using the same. More particularly, this invention relates to aminobenzophenones and methods of using the same in photoactivatable polymerization systems.
Ethylenically unsaturated compounds, and in particular acrylate derivatives, can be polymerized by irradiation with ultraviolet light of wavelength between 200 and 450 nanometers (nm) in the presence of a bimolecular photoinitiating system. The photoinitiating system can include, for example, (1) a benzophenone derivative and (2) a coinitiator or synergist, that is, a molecule which serves as a hydrogen atom donor. The coinitiators or synergists are typically alcohols, tertiary amines or ethers which have available hydrogens attached to a carbon adjacent to a heteroatom.
One commercially available benzophenone derivative useful as a photoinitiator is 4,4xe2x80x2-bis(dimethylamino)benzophenone, also referred to in the art as xe2x80x9cMichler""s Ketonexe2x80x9d. Michler""s Ketone has the following structure: 
While Michler""s Ketone can be useful as a photoinitiator in radiation curing of polymers, it is typically used little in the industry due to its potentially hazardous characteristics.
4,4xe2x80x2-Bis(diethylamino)benzophenone (also referred to as tetraethyl Michler""s Ketone) has been proposed as a possible alternative to Michler""s Ketone due to its lower toxicity. However, this compound does not exhibit good photoinitiating activity and thus has not been widely adopted as an alternative to Michler""s Ketone.
Other Michler""s Ketone derivatives are described, for example, in U.S. Pat. No. 4,507,497 to Reilly, Jr., which is directed to water soluble Michler""s Ketone analogs which include amino groups substituted by R1COOH, in which R1 is an alkylene group having 1 to 8 carbon atoms.
Examples of other commercially available photoinitiators useful in bimolecular photoinitiator systems include benzophenone, 2,4-dimethylbenzophenone, isopropylthioxanthone, and 2,4-diethylthioxanthone. The UV absorption spectrum for these individual photoinitiators, however, do not match-up efficiently with the UV emission spectra of the standard commercially available mercury vapor bulbs.
Thus, commercially viable UV curing processes can require a relatively large amount of initiator and synergist incorporated into the formulation. This can lead to cured articles which contain high levels of residual photoinitiator and synergist, which in turn can result in decreased light fastness and lower resistance to oxidative degradation. In addition, the residual photoinitiator and synergist can be extracted or leach out of the cured article or migrate to the surface of the article. Many times the physical properties of the article are degraded by the presence of the residual photoinitiator and synergist.
It is accordingly an object of the present invention to provide compounds which can be used as photoinitiators in photopolymerization processes. It is also an object of the invention to provide compounds which exhibit useful photoinitiation activity with minimal or no adverse hazardous characteristics. These and other objects of the present invention will become apparent from the following general and detailed description of the invention.
The objects of the present invention are achieved based on the discovery of novel compounds useful in photopolymerization systems. The compounds can display highly active photoinitiation and photopolymerization properties with minimal or no adverse hazardous characteristics. Specifically the compounds can be essentially or substantially non-mutagenic. As is well known in the art, photoinitiator that is not consumed in the photopolymerization reaction may be extracted or leached from the cured product. Therefore a non-mutagenic photoinitiator is especially desirable.
Still further, the compounds can have desirable ultraviolet wavelength absorbance, which in turn can provide advantages when using narrow wavelength lamps. In this regard, advantageously the compounds possess UV spectra with significant absorption bands between 250 and 350 nm and in particular between 290 and 325 nm. Accordingly, the compounds can be irradiated with a narrow wavelength band, high pressure fill UV curing lamp known as an excimer lamp with spectral emphasis in the 250 to 350 nm range, and in particular with its peak emission wavelength at or near 308 nm, as described in U.S. Pat. No. 5,504,391.
Compounds showing a significantly elevated level of reactivity at these wavelengths can be used in considerably lower amounts. For example, when the compounds of the invention are used with an excimer lamp versus a medium pressure mercury lamp, only about one-fourth of the amount of photoinitiator can be required to give equivalent cure speeds. In addition, when the same concentration of photoinitiator is used with both an excimer and a medium pressure mercury lamp the excimer lamp can provide greatly increased cure speeds. Because less photoinitiator is required, less residual photoinitiator can remain in the cured articles, thereby minimizing problems associated with leaching or extraction, decreased light fastness and lower resistance to oxidative degradation.
The compounds of the invention have a structure according to Formula (I) below: 
each A is independently selected from the group consisting of hydrogen, lower alkyl, cycloalkyl, aryl, lower alkanol, lower alkoxy, halogen, sulfonyl, alkylsulfonyl, trihaloalkyl, trihaloalkoxy, trihaloalkylthio, polymerizable moiety, and oligomeric moiety, with the proviso that no more than three A are the same lower alkyl;
each R is independently selected from the group consisting of hydrogen, lower alkyl, cycloalkyl, aryl, lower alkanol, lower alkoxy, halogen, sulfonyl, alkylsulfonyl, trihaloalkyl, trihaloalkoxy, trihaloalkylthio, polymerizable moiety, and oligomeric moiety; and
n is an integer from 1 to 4.
The present invention also provides photopolymerizable compositions which include the compounds of Formula (I) above as a component thereof, as well as methods for the manufacture of the compounds of Formula (I) and methods for the use of the compounds of Formula (I) in photopolymerization systems.